v0.4.0

Big focus on getting xattr behavior fixed for both the Walk/Compare, but also Update().

While working on Update(), paired the behavior more like the bsd-mtree. Including only updating if the value is different.

PRs since v0.3.1: #138
#139
#136
#135
#134
#133
#132
#131
#129
#130
#126
#125
#124
#123
#122
#121
#119
#117
#116